C9orf72 and frontotemporal dementia: As an example, the expansion of GGGGCC (G4C2) repeats within the first intron of chromosome 9 open reading frame 72 (C9orf72) gene is the most frequent familial cause of two devastating neurological disorders: amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).[1] While healthy individuals carry less than 20 hexanucleotide repeat expansions (HRE), abnormal expansion, ranging from 500 to 4000 units of G4C2 repeats, is observed in ALS/FTD patients.[2] However, the mechanism of how HRE drives ALS/FTD pathogenesis remains still elusive.